The invention relates to the field of microelectromechanical components also called MEMS standing for Microelectromechanical Systems. It relates more particularly to MEMS components used in fibre-optic communication devices. The invention relates more specifically to a process for fabricating microelectromechanical components which make it possible to optimize their performance and their manufacturing cost. This process may serve for fabricating various types of optical components which include a moving member moving under the effect of a control command. There may be optical switches, obturators or variable attenuators.
In the rest of the description, the invention will be more particularly described in respect of an optical switch, but it could easily be transposable to an optical obturator or attenuator.
In general, an optical switch receives at least one input optical fibre and at least two output optical fibres. These optical fibres are placed in optical propagation guides oriented very precisely with respect to one another, most generally at 90xc2x0 with respect to one another. The optical switch comprises a mirror which can move in order to intercept the beams propagating in propagation guides. When the moving mirror is in a first position, it allows reflection of the optical beam output by an optical fibre towards a second fibre. When this mirror is in a second position, it does not modify the propagation of the beam output by the first optical fibre, which is therefore transmitted in the optical fibre located in alignment with it.
The movement of this mirror takes place by means of an actuator. Various types of actuators have already been proposed and especially electrostatic actuators, such as in particular that described in document U.S. Pat. No 6,229,640. This type of electrostatic actuator comprises a number of electrodes distributed in two interdigitated combs. These two interdigitated combs partially penetrate one another to form a capacitor thanks to their facing surfaces. Application of an electrical voltage between the two interdigitated combs causes a relative movement of one comb with respect to the other.
Since the mirror is fastened to one of the two combs of electrodes, it moves under the action of this voltage. Positional return takes place when the electrical voltage disappears, owing to the effect of return means which generally consist of one or a number of beams which connect the comb of moving electrodes to the rest of the substrate.
One of the objectives of the invention is to allow the mirror to move using an electrical voltage of a relatively limited value, while obtaining a sufficient excursion of the mirror. However, the use of a voltage of low value causes the facing surface area of the two combs of electrodes to increase.
Moreover, to obtain a movement of the greatest possible amplitude, it is important that the return means do not exert too large a force and that their stiffness be therefore relatively limited. However, this stiffness is determined inter alia by the thickness of the beams which constitute it. Therefore to increase the travel of the mirror, it is tempting to reduce the thickness of the beams of the return means of the actuator.
A problem then arises when it is desired to combine the two aforementioned effects, namely, on the one hand, an increase in the surface area of the electrodes and, on the other hand, a reduction in the thickness of the beams of the return means.
This kind of inconvenience is observed in the microcomponents produced on SOI (Silicon On Insulator)-based substrates. This is because, on SOI substrates, the definition of the electrodes and of the return means of the actuator is produced by etching down to the oxide layer. The electrodes and the return means are then freed by subsequent etching, carried out after the oxide layer has been etched. In this type of component produced from an SOI substrate, the beams of the return means and the electrodes therefore have the same height. To increase the force exerted by the actuator, it is therefore necessary to increase the number of electrodes, which results in a greater consumption of energy by the actuator and a greater occupation of the surface area of the substrate.
It has also been proposed to produce optical switches from a single-crystal silicon substrate, these also being called xe2x80x9cbulkxe2x80x9d switches. Various processes have been developed which depend on the crystallographic orientation of the substrate used. Thus, when the substrate used has an upper face parallel to the (100) plane of the silicon crystal structure, it is possible to carry out, in the same operation, etching of the mirror and of the propagation guides. This is because, thanks to the orientation of the crystal planes which form stop planes for the chemical etching, it is possible to obtain perfect alignment of the propagation guides lying along the same axis, and perfect perpendicularity of the orthogonal propagation guides. However, the thickness of the mirror obtained by this chemical etching depends on the etching time. The precision on the thickness of this mirror is therefore subject to the variations in the conditions under which the etching is carried out. Thus, a slight temperature drift may introduce considerable inaccuracy in the thickness of the mirror.
Wet etching operations are also carried out using substrates whose upper face is parallel to the (110) plane of the silicon crystallographic structure. In this case, the chemical etching stop planes correspond to the vertical sidewalls of the mirror, thereby making it possible to achieve very good precision on the thickness of the mirror.
However, in this situation, it is necessary to produce the propagation guides in a second phase, since the crystallographic axes do not coincide with the directions of these propagation guides. It is therefore necessary to produce them by a subsequent step, generally requiring the use of dry etching, of the reactive ion etching or RIE type.
One of the objectives of the invention is therefore to allow optical components to be produced from single-crystal silicon with a minimum number of steps.
Document U.S. Pat. No. 6,150,275 has described a process for producing microstructures from single-crystal silicon, the (111) crystallographic planes of which are parallel to the principal plane of the substrate. The process described in that document consists in linking dry etching steps for defining the contours of a microstructure on the substrate. This process is continued by a chemical etching step which makes it possible to free the structure predefined by the dry etching.
The invention therefore relates to a process for fabricating a microelectromechanical optical component which is produced from a silicon substrate. Such an optical component generally comprises:
at least two optical propagation guides, especially intended to receive optical fibres;
a wall which can move with respect to the propagation guide;
an electrostatic actuator capable of causing the moving wall to move with respect to the rest of the substrate, the said actuator comprising:
facing electrodes which can move with respect to each other, some of the electrodes being mechanically linked to the moving wall, the other electrodes being fastened to the rest of the substrate;
return means formed by at least one beam produced in the substrate and opposing the movement of the electrodes with respect to one another.
In accordance with the invention, the substrate used is made of single-crystal silicon, the (111) planes of which are parallel to the planes of the substrate. This process firstly comprises a first series of deep reactive ion etching steps during which the heights of the moving wall, of the electrodes of the actuator, and of the beams of the return means of the actuator are defined with different values. This process continues with a second wet etching step, making it possible to free the moving wall, the electrodes and the beams of the actuator from the rest of the substrate.
In other words, it is possible to produce a mirror having a height substantially greater than the height of the electrodes and of the beams of the return means of the actuator. It is thus possible to optimize the ratio of the various heights of the members of the component. Thus, the electrode etching height will be chosen to be greater than that of the beams of the return means. In this way, the height of the electrodes, and therefore the force which is exerted between the two sets of electrodes, is increased for the same control voltage. At the same time, by reducing the thickness of the beams of the return means, the stiffness of the latter is reduced, thereby making it possible to increase the excursion of the mirror. This possibility is provided by differentiated deep reactive ion etchings whereby the contours of the electrodes or of the walls of the return means are defined.
The depth of the deep reactive ion etching used to define the mirror may be chosen in various ways. Thus, this depth may be chosen so that the optical fibre can be completely included within the propagation guide, thereby facilitating the subsequent encapsulation phases.
The depth of this first etching may also be less, provided that it is sufficient to ensure interception by the mirror of the beam output by the optical fibres. In this case, since the reflecting region of the mirror lies close to the upper face of the substrate, it exhibits better planarity and verticality, and therefore better reflecting properties.
In practice, before the deep reactive ion etching steps, a masking step for defining the subsequent position of the optical propagation guides of the moving wall, of the electrodes and of the return means of the actuator is carried out. The masking may for example take place by depositing an SiO2 layer, for example by PECVD (Plasma Enhanced Chemical Vapour Deposition) techniques. This layer is then configured by a conventional method consisting of lithography followed by etching.
Advantageously in practice, the process comprises two successive deep reactive etching steps, namely:
a first deep reactive ion etching step during which the heights of the moving wall, of the electrodes and of the return means of the actuator are defined; and
a second deep reactive ion etching step during which the volumes from which the subsequent wet etching step may be initiated are defined.
In practice, between these two deep reactive ion etching steps, the following are carried out:
firstly, conformal deposition of an SiO2 layer in the regions etched by the first etching step; and
then removal of this SiO2 layer from the bottom of the initially etched regions.
The term xe2x80x9cconformalxe2x80x9d deposition is understood to mean that the deposition is carried out over the entire visible surface of the substrate, with a constant thickness.
It is in the bottom of these regions that the subsequent step of reactive ion etching will take place, allowing the volumes from which the wet etching step can start to be defined.
After the wet etching step, a metallization step is carried out, which allows the moving wall to be made reflecting. This step also allows the electrodes to be metallized, thereby allowing the control voltage to be applied.
According to another characteristic of the invention, during the deep reactive ion etching steps, a number of small protection beams, located on either side of the beams forming part of the return means of the actuator, are defined. These small protection beams are joined to the beams of the return means via linking portions of small dimensions. These small protection beams are then freed from the rest of the substrate during the wet etching step. These small protection beams, lying on each side of the beams of the return means, allow the loading effects during deep etching of the substrate to be increased.
This is because, to obtain a sufficient excursion of the mirror, it is necessary for the return means to deform relatively substantially. Certain parts of the beams of the return means therefore move by a distance approximately equivalent to that travelled by the mirror.
The beams of the return means therefore lie in the widely open spaces. However, it is known that the depth of deep reactive ion etching depends on the surface area of the etched features. Features of smaller surface area are etched less deeply than features of larger dimensions. This phenomenon, known by the name xe2x80x9cload effectxe2x80x9d, is used when defining the beams of the return means in order to make two factors which seem a priori contradictory compatible, these being, on the one hand, the desired precision on the dimensions of the beams of the return means and, on the other hand, the width of the opening in which these beams are located.
This is because the cross section of the beams of the return means is a dominant parameter in determining the stiffness of these return means. To obtain a precise stiffness, it is therefore necessary to control the cross section of these beams of the return means. To do this, the precise contour of the beams of the return means is defined by features of very small width. Outside these features, the small protection beams, which will be more exposed to the deep reactive ion etching in the relatively open space which surrounds the beams of the return means, are produced. The load effect phenomena during deep reactive ion etching therefore are observed in the narrow features separating the beams of the return means from the small protection beams.
The reactive ion etching depth will be different on the two sides of the small protection beams. Thus, on that side of the small protection beams facing the open space in which the return means will move, the etching will be relatively deep. On the other hand, on that side of the small protection beams facing the beams of the return means, this etching will be substantially shallower, so as to produce a beam of small thickness, and therefore having a more controlled and therefore optimized stiffness. The use of small protection beams therefore makes it possible to create an additional etching depth level and to control the cross section and therefore the stiffness of the beams of the return means.
Since the small protection beams are joined to the beam of the return means by linking regions of very small dimensions, they play virtually no part in determining the stiffness of the beam of the return means, but act as weights distributed over the length of this beam.
The process according to the invention can be employed for obtaining various types of optical components. These may be components of the variable attenuator or obturator type, which possess two collinear optical propagation guides.
There may also be components of the switch type incorporating two pairs of collinear propagation guides, one pair being perpendicular to the other. In this case, the mirror moves in a plane at 45xc2x0 with respect to the propagation guide.
By optimizing the compromise between the stiffness of the return means and the volume of electrodes, an actuator of substantially smaller size than that of the prior art is obtained. It is therefore possible to bring the optical propagation guides closer together in a manner sufficient to produce multiple switches based on elementary cells in the form of matrices, by increasing the concentration and the integration density of the elementary switching cells.